The present invention relates to a method for producing a positive electrode active material for an alkaline storage battery.
Secondary batteries are mounted on most of portable apparatuses, such as cellular phones and laptop computers. Under such circumstances, development of a secondary battery having a higher capacity has been strongly desired. The positive electrode has been improved as discussed below to attain the higher capacity of alkaline storage batteries.
Both a sintered positive electrode and a paste type positive electrode are applicable for the alkaline storage battery. The sintered positive electrode has a substrate having pores of approximately 10 xcexcm in diameter. The substrate is obtained by sintering a nickel powder and a core material such as a perforated metal sheet, and has a small porosity of approximately 80%. This substrate is accordingly filled with a relatively small quantity of the active material. The paste type positive electrode, on the other hand, has a foamed nickel substrate having pores of approximately 500 xcexcm, where the pores are communicating each other and arranged in a three-dimensional manner. The foamed nickel substrate has a large porosity of approximately 95%. This substrate is accordingly filled with a relatively large quantity of the active material. Namely the paste type positive electrode has a higher capacity.
The electrical conductivity of nickel hydroxide, which is the active material of the paste type positive electrode, varies with a variation in oxidation number of nickel: that is, high conductivity for the large oxidation number and low conductivity for the small oxidation number. The oxidation of nickel hydroxide in charging process of a battery thus proceeds smoothly, but the reduction in discharging process does not proceed smoothly, due to the lowered electrical conductivity in the terminal stage of the discharging process. This causes an insufficient discharge. A conductive agent, such as a cobalt compound, is added to the active material, with a view to enhancing the electrical conductivity in the positive electrode and ensuring the sufficient discharge.
In the case cobalt hydroxide is added to the active material, the first charging after the manufacture of the battery causes a cobalt oxyhydroxide having a good conductivity to deposit on the surface of the nickel hydroxide as the active material. This ensures the favorable conductive networks as is explained in Japanese Laid-Open Patent Application Sho 61-74261. A cobalt oxyhydroxide is stable within a standard voltage range of the battery and keeps the conductive networks.
In the alkaline storage batteries, the negative electrode generally has a greater capacity than the capacity of the positive electrode. The residual non-charged capacity of the negative electrode under the full charged condition of the positive electrode is referred to as the charge reservoir, and the residual charged capacity of the negative electrode under the full discharged condition of the positive electrode is referred to as the discharge reservoir.
When the battery is excessively charged, the reaction as defined below occurs at the positive electrode to produce oxygen:
OH31 xe2x86x921/2H2O+1/4O2+exe2x88x92
Oxygen reacts with the hydrogen absorbed in the negative electrode and is consumed like below:
MH (metal hydride)+1/4O2xe2x86x92M (alloy)+1/2H2O
M+H2O+exe2x88x92xe2x86x92MH+OHxe2x88x92
The hydrogen storage alloy of the negative electrode hardly absorbs hydrogen in the terminal stage of a charging process of a battery. The presence of the alloy that has not yet absorbed hydrogen as the charge reservoir effectively depresses the generation of gaseous hydrogen, which enables the battery to be sealed.
The following describes the discharge reservoir in the paste type positive electrode including nickel, which is obtained by adding cobalt hydroxide as a conductive agent to nickel hydroxide functioning as the active material. The initial charging of the battery having this positive electrode changes cobalt hydroxide to a cobalt oxyhydroxide. The electrical quantity stored in the negative electrode during this process becomes a part of the discharge reservoir. The capacity of nickel hydroxide is 289 mAh/g, and the capacity of cobalt hydroxide is 288 mAh/g. In the case cobalt hydroxide is used at a rate of 10% by weight of nickel hydroxide, the discharge reservoir obtained is about one tenth of the capacity of the positive electrode.
The oxidation number of nickel in nickel hydroxide is initially 2 but rises to approximately 3.2 by charging of the battery, so that nickel hydroxide is changed to a nickel oxyhydroxide. The discharging of the battery is concluded when the oxidation number of nickel decreases to approximately 2.2. The non-discharged nickel oxyhydroxide thus remains to give the discharge reservoir of about two tenths of the capacity of the positive electrode. The nickel-metal hydride storage battery accordingly has the total discharge reservoir of about three tenths of the capacity of the positive electrode.
The adequate quantity of the discharge reservoir is at most about one tenth of the capacity of the positive electrode. Namely the capacity corresponding to about two tenths of the capacity of the positive electrode are excessive in the negative electrode. In other words, the prior art battery includes a specific quantity of the hydrogen storage alloy that does not contribute to charging and discharging. Regulating the quantity of the discharge reservoir to the appropriate level desirably reduces the required quantity of the expensive hydrogen storage alloy and gives a battery of high energy density at a low manufacturing cost.
From these viewpoints, the positive electrode of the battery disclosed in Japanese Laid-Open Patent Application Sho 60-254564 includes nickel hydroxide, cobalt, and a nickel oxyhydroxide required for oxidation of cobalt. This proposed battery has the reduced discharge reservoir due to the oxidation of cobalt. The positive electrodes of the batteries disclosed in Japanese Laid-Open Patent Applications Hei 4-26058 and Hei 8-148145 include a particulate nickel hydroxide with a cobalt oxyhydroxide carried thereon.
The battery disclosed in Japanese Laid-Open Patent Application Hei 11-219701 seems to attain the greatest effect of reducing the discharge reservoir, among the various prior art batteries. The positive electrode of this proposed battery includes a first active material, which comprises a particulate nickel hydroxide with a cobalt oxyhydroxide carried thereon, and a second active material, which comprises a particulate nickel oxyhydroxide with a cobalt oxyhydroxide carried thereon. The weight ratio of the first active material to the second active material ranges from 90/10 to 60/40. Some methods for obtaining such a particulate nickel hydroxide with a cobalt oxyhydroxide carried thereon have been disclosed in Japanese Laid Open Patent Hei 10-74512 and Hei 11-329425.
In the positive electrode of the battery disclosed in Japanese Laid Open Patent Hei 11-219701, however, the oxidation number of nickel in the nickel oxyhydroxide of the second active material is not specified. The quantity of the discharge reservoir in the negative electrode depends upon not only the weight ratio of the first active material to the second active material but the oxidation number of nickel in the nickel oxyhydroxide of the second active material. Namely the adequate quantity of the discharge reservoir is unknown in the battery disclosed in Japanese Laid Open Patent Hei 11-219701. Further, there are disclosed no method for producing a positive electrode active material, which is capable of oxidizing the cobalt hydroxide portion and nickel hydroxide portion of the raw material to a desired oxidation state.
Conventionally, particulate nickel hydroxide with a cobalt oxyhydroxide carried thereon is prepared by dispersing a nickel powder having cobalt hydroxide carried thereon in an aqueous alkaline solution. If, however, the alkali concentration of the aqueous alkaline solution is high, cobalt hydroxide of the raw material is undesirably dissolved in the aqueous alkaline solution or precipitated again. In addition, a rapid structure change occurs in the cobalt hydroxide portion of the raw material and causes the raw material to partially liberate any cobalt compounds. These cobalt compounds thus liberated are very fine and hence adhere to the inner wall of the reaction bath. Accordingly, the quantity of cobalt in the active material varies. If the alkali concentration of the aqueous alkaline solution is low, on the other hand, oxidation of the cobalt hydroxide portion does not proceed until the oxidation number of cobalt exceeds 3 particularly when the temperature is low. This results in a production of a cobalt oxyhydroxide of a low oxidation state, which exhibits low electrical conductivity. Use of such an active material causes the capacity of the battery to lower.
The present invention is directed to a method for producing a positive electrode active material for an alkaline storage battery, comprising a first oxidation treatment of a raw material powder, which comprises a nickel hydroxide solid solution, that is, a hydroxide comprising a plurality of metals including Ni as a major component, and cobalt hydroxide, to oxidize the cobalt hydroxide to a cobalt oxyhydroxide; and a second oxidation treatment of a powder having passed through the first oxidation treatment to oxidize the nickel hydroxide solid solution to a nickel oxyhydroxide solid solution, that is, an oxyhydroxide comprising a plurality of metals including Ni as a major component.
The present invention is also directed to a method wherein the raw material powder is a nickel hydroxide solid solution powder having cobalt hydroxide carried thereon, or a mixture comprising a nickel hydroxide solid solution powder and a cobalt hydroxide powder.
The present invention is also directed to a method wherein the first oxidation treatment is a process where the raw material powder with an aqueous alkaline solution is heated and dried in an atmosphere having oxygen while being stirred, and the second oxidation treatment is a process where the powder having passed through the first oxidation treatment is dispersed in water or an aqueous alkaline solution and mixed with an oxidizing agent.
It is preferable that, in the first oxidation treatment, the aqueous alkaline solution has a normality of 1N or more and contains at least one selected from the group consisting of sodium hydroxide and potassium hydroxide, and in the second oxidation treatment, the aqueous alkaline solution contains at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.
It is also preferable that, in the first oxidation treatment, the cobalt hydroxide is oxidized until an oxidation number of cobalt thereof becomes more than 3, and in the second oxidation treatment, the nickel hydroxide solid solution is oxidized until an oxidation number of nickel thereof becomes not less than 2.1 and less than 2.2. In this case, the resultant product can be used as an active material that is capable of making the quantity of the discharge reservoir adequate even when used alone.
The present invention is also directed to a method wherein the raw material powder is a nickel hydroxide solid solution powder having xcex2-cobalt hydroxide carried thereon, the first oxidation treatment is a process where the raw material powder is heated to 80 to 160xc2x0 C. with heated air, or a process where the raw material powder is dispersed in water or an aqueous alkaline solution and mixed with an oxidizing agent, to give a nickel hydroxide solid solution powder having cobalt oxyhydroxide of a low oxidation state carried thereon, and the second oxidation treatment is a process where the powder having passed through the first oxidation treatment is dispersed in an aqueous alkaline solution of 30 to 80xc2x0 C. having a normality of 1N or more and mixed with an oxidizing agent to give a nickel oxyhydroxide solid solution powder having a cobalt oxyhydroxide of a high oxidation state carried thereon.
In this method, it is preferable that an oxidation number of cobalt in the cobalt oxyhydroxide of a low oxidation state is more than 2 and not more than 3, and an oxidation number of cobalt in the cobalt oxyhydroxide of a high oxidation state is more than 3.
The electrical conductivity in the compressed form of the cobalt oxyhydroxide of a low oxidation state is, for example, approximately 10xe2x88x927 to 10xe2x88x925 S/cm. The electrical conductivity in the compressed form of the cobalt oxyhydroxide of a high oxidation state is preferably higher than that of the former, for example, approximately 10xe2x88x922 to 1.0 S/cm.
It is also preferable that, in the second oxidation treatment, the aqueous alkaline solution contains at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, and lithium hydroxide.
The present invention is further directed to a method for producing a positive electrode active material for an alkaline storage battery, comprising a process where a nickel hydroxide solid solution powder having a-cobalt hydroxide carried thereon is dispersed in an aqueous alkaline solution of 25 to 80xc2x0 C. having a pH value of not more than 10 and mixed with an oxidizing agent to give a positive electrode active material comprising a nickel oxyhydroxide solid solution powder having a cobalt oxyhydroxide carried thereon.
It is preferable that the nickel hydroxide solid solution contains at least one element selected from the group consisting of Co, Zn, Cd, Ca, Sr, Mg, Mn, and Al.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.